In the present-day shadow-mask color-picture tubes, the three electron beams produced by such an electron-gun system must be adjusted so that all three beams intersect at the same opening in the shadow mask. This adjustment is called "convergence adjustment". The simplest method is to move the three electron beams individually, as is described, for example, in German Patent DE-OS NO. 27 22 477. With an assembly as disclosed in that publication, it is easy to move each electron beam virtually independently of each of the other two beams. Besides this single-beam movement, however, it is also possible to move all three electron beams jointly in the electron-beam plane to adjust purity, and perpendicular to the electron-beam plane to adjust the raster. However, the overall structure is rather complicated, unstable and costly.
German Patent DE-OS No. 26 12 607 discloses another device for adjusting the convergence, purity and raster of a color-picture tube, which device is located inside the tube neck. A circular wire or strip ring is mounted in the region of the electron-gun system and so magnetized from outside that the electron beams are adjusted as desired. Through the use of a single circular wire ring which is closed except for an air gap, the construction described is very simple and stable. However, it is not possible to move the electron beams independently of each other.